1. Field of the Invention
The present invention relates to an antenna and an antenna unit both for use mainly in mobile communications. More specifically, the present invention relates to an antenna for a base station in mobile communications, and an antenna unit using the antenna.
2. Description of the Background Art
With reference to FIGS. 36 through 40, one example of a conventional antenna is described below. FIG. 36 is an illustration showing the configuration of a monopole antenna described in Japanese Patent Laid-Open Publication No. 2001-308630. The antenna includes a top conductor 111, a ground conductor 112, side conductors 113, an antenna element 114, and a power supply point 115. The antenna has a feature of having bi-directional directivity on a horizontal plane. The top conductor 111 has there parts 111a, 111b, and 111c, with the part 111b located at the center of the conductor 111 being implemented by a linear conductor. The top conductor 111, the ground conductor 112, and the side conductors 113 form an antenna box having the shape of a rectangular parallelepiped. The antenna box has two openings 116 and 117 on the top. The power supply point 115 is located at the center of the ground conductor 112. The antenna element 114 is connected at one end to the power supply point 115. Furthermore, the antenna element 114 is mechanically or electrically connected at the other end to the center point of the ground conductor 111b by, for example, soldering. When a coordinate system is set as illustrated in FIG. 36 by taking the power supply point 115 as an origin, the antenna has a symmetric structure with respect to both of a Z-Y plane and a Z-X plane.
With reference to FIG. 37, the operation of the antenna illustrated in FIG. 36 is described below. Excitation of electric waves occurs at the antenna element 114, from which an electric wave having a frequency of f0 is emitted. The electric waves are emitted through two openings 116 and 117 to the outside of the antenna box. These two openings 116 and 117 are located symmetrically to the antenna element 114, which is an emitting source. Therefore, the distance from the antenna element 114 to the opening 116 is equal to the distance from the antenna element 114 to the opening 117. Also, as illustrated in FIG. 37A, the direction of the electric field excited at the opening 116 is opposed to the direction thereof excited at the opening 117. Here, for the sake of convenience in description, consider a case in which the electric fields excited at these openings 116 and 117 are replaced by magnetic flows. In this case, as illustrated in FIG. 37B, it can be assumed that two linear magnetic flow sources B1 and B2 are located at the openings 116 and 117, respectively, in parallel to the Y axis. These linear magnetic flow sources B1 and B2 have the same amplitude, but are oriented to opposite directions. Here, the electric waves emitted from the antenna can be considered as being emitted from these two magnetic flow sources B1 and B2. That is, emission from the antenna can be regarded as emission from an array of the magnetic flow sources B1 and B2.
As illustrated in FIG. 37B, the magnetic flow sources B1 and B2 are located symmetrically with respect to the Z-Y plane. For this reason, the electric waves emitted from the magnetic flow sources B1 and B2 are equal in amplitude and opposite in phase to each other on the Z-Y plane, and therefore are cancelled by each other. With this, no electric wave is emitted on the Z-Y plane. On the Z-X plane, the electric waves emitted from the magnetic flow sources B1 and B2 are equal in phase to each other in one direction. In that direction, the electric wave from the antenna is intensified. For example, when an interval between the magnetic flow sources B1 and B2 is xc2xd a wavelength in free space, two electric waves are equal in phase to each other at an arbitrary point on the X axis. Therefore, the electric wave from the antenna is intensified in both of the +X direction and the xe2x88x92X direction. That is, the antenna illustrated in FIG. 36 has a bi-directional directivity in the X direction.
As such, according to the antenna illustrated in FIG. 36, an effect of an antenna array can be achieved only by a single antenna element, and a directivity can be provided to the antenna. Furthermore, when the openings 116 and 117 are made longer in the Y direction, for example, the magnetic flow sources also become longer. Therefore, emission in the X direction is reduced, thereby producing larger gain. As such, gain can be adjusted depending on the length of the openings.
In general, the size of conductive members that construct the antenna is finite. Therefore, the electric wave is diffracted at the end portion of each conductive member. Therefore, precisely speaking, the electric wave emitted from the antenna is a sum of an electric wave emitted from the antenna element and diffracted waves at the end portions of the respective conductive members. The same goes for the antenna illustrated in FIG. 36. That is, the electric wave is diffracted at every end portion and every refraction point of the conductors 111, 112, and 113. Particularly, since the top conductor 111 and the openings 116 and 117 are located on a same plane, the electric wave emitted from the antenna is greatly influenced by a diffracted wave at the end of the top conductor 111. Thus, the directivity of the antenna illustrated in FIG. 36 is varied by the number or locations of the openings 116 and 117 as well as the size and shape of the conductors 111, 112, and 113.
By way of example only, the characteristics of a prototype antenna illustrated in FIG. 38 are described. In FIG. 38, when a free space wavelength of xcex0 is taken as a reference, the ground conductor 112 is shaped like a square whose side is 0.836xc3x97xcex0 each, and the height of each side conductor 113 is 0.0836xc3x97xcex0. The top conductor 111b at the center is a linear conductor being located in parallel to the Y axis and having a length of 0.836xc3x97xcex0. Both ends of the top conductor 111b are electrically connected to the side conductors 113. The top conductors 111a and 111c are each shaped like a rectangle having two sides each being parallel to the X axis and having a length of 0.209xc3x97xcex0 and the other two sides each being parallel to the Y axis and having a length of 0.836xc3x97xcex0. These top conductors 111a and 111c are connected to the side conductors 113. The two openings 116 and 117 are also each shaped like a rectangle having two sides each being parallel to the X axis and having a length of 0.209xc3x97xcex0 and the other two sides each being parallel to the Y axis and having a length of 0.836xc3x97xcex0. These openings 116 and 117 are located adjacently to each other so as to sandwich the conductor 111b therebetween. Therefore, the antenna box has a symmetric structure with respect to both of a Z-Y plane and a Z-X plane. The antenna element 114 is a linear conductor having the element length of 0.0835xc3x97xcex0. One end of the antenna element 114 is electrically connected to the midpoint of the top conductor 111b. 
FIG. 39 is a graph showing VSWR (Voltage Standing Wave Ratio) characteristics with respect to a power supply line of 50xcexa9 as input impedance characteristics of the prototype antenna illustrated in FIG. 38. In FIG. 39, the horizontal axis represents frequencies standardized with a center frequency of f0. Frequencies of f1 and f2 are a minimum frequency and a maximum frequency, respectively, both of whose VSWR is 2 or less. According to FIG. 39, a frequency band whose VSWR is 2 or less is ((f2xe2x88x92f1)/f0)=18.2%. Therefore, the prototype antenna illustrated in FIG. 38 has improved impedance characteristics with less reflection loss over a wide band.
FIG. 40 is an illustration showing an mission directivity of the prototype antenna illustrated in FIG. 38 at the center frequency of f0. In the drawing, a scale of the radiation directivity is in units of 10 dB and its unit is dBi with reference to emission power of a point wave source. As can be seen from FIG. 40, the antenna illustrated in FIG. 38 suppresses emission of an electric wave in the Y direction, and has a bi-directional directivity in the X direction. Therefore, this antenna can outstanding characteristics in a long interior space, such as a corridor.
Moreover, the height of the antenna element 114 of the prototype antenna is 0.0835xc3x97xcex0, which is lower than the height of a normal xc2xc-wavelength antenna element. Therefore, even when the antenna cannot be embedded in a ceiling, the antenna does not protrude much from the ceiling, and is thus inconspicuous. For this reason, this antenna does not disturb the outer look of the ceiling, and therefore is preferable. Also, the antenna box has a symmetric structure with respect to a predetermine plane. Therefore, the directivity of the antenna can be symmetrical to that plane. As described above, it is possible to achieve a high-performance antenna having a desired bi-directional directivity with a small and simple structure.
The above-described conventional antenna, however, has a drawback that its directivity cannot be biased to a specific direction. That is, the conventional antenna has a bi-directional directivity on a horizontal plane, and is suitable for covering a long-shaped space, such as a corridor, but does not have a directivity biased to a specific direction. This poses a problem when the antenna is used in a room, for example. Now consider a case in which an antenna is placed between a receiver of a communications system and another communications system, and the frequency used by the former communications system is close to that used by the latter communications system. In this case, an electric wave emitted from the antenna to the receiver of the former communications system is an interfering wave to the latter communications system. Therefore, power of the electric wave emitted from the antenna is required to be small, to some extent. With the power of the emitted electric wave being small, however, the electric wave received by the receiver also becomes small, thereby causing a communicable area to be narrowed. For this reason, the conventional antenna incapable of biasing the direction of emitting an electric wave is not adequate in an environment in which a plurality of systems using frequency bands close to each other are closely located to each other (in a room, for example).
Furthermore, the conventional antenna has another drawback that its directivity is fixed. That is, since the directivity of the antenna is determined based on the shape of the antenna and the frequency to be used, the directivity of the antenna cannot be changed after the antenna is installed unless the antenna is reoriented. However, when a communications system is installed in a place close to an antenna that has already been installed, the directivity of the antenna that has already been installed may be desired to be changed in some cases. Moreover, if the directivity of the antenna can be dynamically controlled based on a receiver""s location varying with time, highly-reliable communications with less noise can be achieved by making the most of the characteristics of the antenna.
Therefore, an object of the present invention is to provide an antenna capable of biasing a bi-directional radiation directivity to a desired direction and controlling the directivity after installment, and an antenna unit using the antenna.
The present invention has the following features to attain the object mentioned above.
A first aspect of the present invention is directed to an antenna with a directivity, including: a ground conductor; at least two power supply sections placed on a surface of the ground conductor; at least two antenna elements connected one-to-one to the power supply sections; a top conductor opposed to the ground conductor across the antenna elements; side conductors surrounding a space including the antenna elements, being located apart from the antenna elements, and forming, together with the top conductor and the ground conductor, an antenna box having at least two openings; and a power supply control section for controlling signals passing through between an external connecting terminal and the power supply sections. According to the first aspect, it is possible to provide a small, slim, simply-structured antenna capable of biasing the directivity to a desired direction and controlling the directivity even after installation.
In this case, the power supply control section may switch the power supply sections for connection to the external connecting terminal. With this switching function, the directivity of the antenna can be biased to a specific direction, and also be controlled after installation.
Furthermore, the power supply control section may have at least either one of a function of combining signals supplied by the power supply sections for output to the external connecting terminal and a function of separating a signal supplied through the external connecting terminal for output. With such signal combining/separating function, the directivity of the antenna can be biased to a specific direction, and also be controlled after installation.
Still further, the power supply control section may include a phase adjusting section for changing a phase of the signal or an amplitude adjusting section for changing an amplitude of the signal, which is located at a point on a route from the external connecting terminal to one of the power supply sections. As such, by using the phase adjusting section or the amplitude adjusting section, the phase or amplitude of each of the signals of the antenna elements is appropriately adjusted. With this, the directivity of the antenna can be biased to a specific direction, and also be controlled after installation.
Still further, only one said top conductor may be provided to the antenna. Also, all of the antenna elements may be placed in a space between the top conductor and the ground conductor. As such, signals supplied to the antenna elements are controlled by using the power supply control section. With this, the characteristics of electric waves emitted from two or more openings formed on the antenna box can be switched. Moreover, the directivity of the antenna can be biased to a specific direction, and also be controlled after installation.
Still further, at least two of said top conductors may be provided to the antenna. Also, and each of the antenna elements may be placed in a space between each of the top conductors and the ground conductor. As such, signals supplied to the antenna elements are controlled by using the power supply control section. With this, the characteristics of electric waves emitted from two or more openings formed on the antenna box can be switched. Moreover, the directivity of the antenna can be biased to a specific direction, and also be controlled after installation. Still further, the top conductor is provided to each of the antenna elements. Therefore, when one antenna element is selected for operation, the top conductor(s) of the other antenna element(s) acts as a reflector(s). Thus, the directivity of the antenna can be further biased to a specific direction.
Still further, the top conductor and the antenna elements may be electrically connected to each other. With such electrical connection, the impedance of the antenna can be stabilized. Therefore, the characteristics of the antenna can also be stabilized.
Still further, the antenna box, and shapes and locations of the openings may be symmetrical with respect to a first plane which is perpendicular to the ground conductor. With this, the directivity of the antenna can be made symmetrical with respect to the first plane.
In this case, the power supply sections may be placed symmetrically with respect to the first plane. With this, when the power supply sections are connected to the antenna elements, these antenna elements are placed symmetrically with respect to the first plane. Therefore, the directivity of the antenna can be made symmetrical with respect to the first plane.
Still further, the antenna box, and shapes and locations of the openings may be symmetrical with respect to a second plane which is perpendicular to both of the ground conductor and the first plane. With this, the directivity of the antenna can be made symmetrical with respect to the second plane.
In this case, the power supply sections may be placed symmetrically with respect to the first plane and the second plane. With this, when the power supply sections are connected to the antenna elements, these antenna elements are placed symmetrically with respect to the first and second planes. Therefore, the directivity of the antenna can be made symmetrical with respect to the first and second planes.
Alternatively, the ground conductor can be in a shape of a rectangle. With this, it is possible to install the antenna on a ceiling, for example, so as to conform to squares often designed on the ceiling or the shape of a room in order to prevent the antenna from being conspicuous.
Still alternatively, the ground conductor can be in a circular-like shape. With this, it is possible to install the antenna on a ceiling, for example, in a desired direction irrespectively of the squares often designed on the ceiling or the shape of the room.
Still further, the antenna may further include at least one matching conductor which is accommodated in the antenna box, is electrically connected to the ground conductor, and is placed apart from the antenna elements. In this case, at least one matching conductor may be electrically connected to the antenna elements or the top conductor. With the matching conductor being provided, it is possible to match the impedance of each antenna element and the impedance of each power supply line, thereby efficiently supplying power.
Still further, the antenna may further include at least one isolation adjusting conductor which is accommodated in the antenna box and is connected at one end to the ground conductor. In this case, at least one isolation adjusting conductor is connected to the top conductor. With this, it is possible to provide an antenna having desired isolation characteristics and capable of controlling the radiation directivity.
Still further, the antenna may further include a dielectric material which is accommodated in the antenna box, and the dielectric material has a dielectric constant higher than a dielectric constant of air. With this, the wavelength is reduced in the dielectric material. Therefore, the antenna can be made smaller and slimmer without deteriorating the characteristics, such as the directivity.
In this case, the antenna box may be entirely filled with the dielectric material. With this, air full of dust or moisture can be prevented from entering inside of the antenna box. Therefore, deterioration in antenna characteristics due to such air can be prevented.
Still further, the top conductor and the ground conductor may be formed by metal foils laminated to a dielectric plate, and the side conductors are formed with via holes. By manufacturing an antenna with the use of a plate processing technology such as etching, the accuracy in manufacturing the antenna can be improved. Also, cost incurred in mass production of antennas can be reduced.
Alternatively, the dielectric material may occupy part of the inside of the antenna box, and may cover the openings. With the openings being covered by the dielectric material, air full of dust or moisture can be prevented from entering inside of the antenna box. Therefore, deterioration in antenna characteristics due to such air can be prevented.
Still further, the antenna may further include an opening control section for changing a size of at least one of the openings. By changing the size of the opening, the radiation directivity of the antenna can be changed. Also, by combining a control of the radiation directivity by the opening control section and a control thereof by the power supply control circuit section, it is possible to easily achieve a desired radiation directivity.
Still further, the power supply control section may be placed on the ground conductor inside of the antenna box. With this, the antenna can be made small.
In this case, the antenna may further include a shield material made of metal which is accommodated in the antenna box. Also, the power supply control section may be placed in a space shielded by the ground conductor and the shield material. With this, the antenna can be made small. Also, it is possible to reduce the influence of electric fields occurring inside of the antenna box on the operation of the power supply control section.
Still further, the ground conductor may have a concave portion oriented inwardly to the antenna box. Also, the power supply control section may be placed in the concave portion of the ground conductor outside of the antenna box. With this, the antenna can be made small. Also, it is possible to reduce the influence of electric fields occurring inside of the antenna box on the operation of the power supply control section.
In this case, the antenna may further include a shield material made of metal which covers the concave portion of the ground conductor. Also, the power supply control section may be placed in a space shielded by the concave portion of the ground conductor and the shield material. With this, the antenna can be made small. Also, it is possible to reduce the influence of electric fields occurring inside of the antenna box on the operation of the power supply control section.
A second aspect of the present invention is directed to an antenna unit including an antenna with a directivity, including: a ground conductor; at least two power supply sections placed on a surface of the ground conductor; at least two antenna elements connected one-to-one to the power supply sections; atop conductor opposed to the ground conductor across the antenna elements; side conductors surrounding a space including the antenna elements, being located apart from the antenna elements, and forming, together with the top conductor and the ground conductor, an antenna box having at least two openings; a power supply control section for controlling signals passing through between an external connecting terminal and the power supply sections; and a radio circuit for supplying the antenna connecting terminal with a radio signal received from an antenna control device externally provided, and transmitting a radio signal output from the antenna connecting terminal to the antenna control device. With this, it is possible to provide an antenna unit including a small, slim, simply-structured antenna capable of biasing the directivity to a desired direction and controlling the directivity even after installation.
Still further, the radio circuit may include a converter circuit for converting an optical signal to an electrical signal and an electrical signal to an optical signal, and may perform optical communications with the antenna control device. With this, it is possible to provide an antenna unit including a small, slim, simply-structured antenna capable of biasing the directivity to a desired direction and controlling the directivity even after installation. This antenna unit also enables optical communications with the antenna control device.
Still further, only one said top conductor may be provided to the antenna, and all of the antenna elements may be placed in a space between the top conductor and the ground conductor. Alternatively, at least two of said top conductors may be provided to the antenna, and each of the antenna elements may be placed in a space between each of the top conductors and the ground conductor. As such, signals supplied to the antenna elements are controlled by using the power supply control section. With this, it is possible to provide an antenna unit including an antenna capable of switching the characteristics of electric waves emitted from two or more openings formed on the antenna box. The antenna is also capable of biasing the directivity of the antenna to a desired direction, and controlling the directivity after installation.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.